1. Field of the Invention
The invention relates to a high voltage inverter device such as a switching regulator, an inverter or the like used in a high voltage power supply unit, a power supply unit for discharge or the like.
2. Description of the Related Art
A high voltage inverter device using, as an input voltage, a DC voltage or a voltage composed of a DC component with a pulsating current superposed thereon, switching the input voltage to apply an exciting current to an excitation winding on a primary side of a transformer and output a high voltage from an output winding on a secondary side thereof is in heavily used as a high voltage power supply unit.
For example, atmospheric pressure plasma is applied to various industrial products as a means for surface treatment for improvement of surface quality, removal of contamination or the like. In the case where adhesion, printing, coating or the like is applied to a resin or the like, performing pre-treatment using the atmospheric pressure plasma can improve the wettability thereof.
In order to generate the atmospheric pressure plasma, a high voltage is required, and the high voltage needs to be safely obtained by an inverter device.
In an alternating-current inverter device generating a high voltage of several KV or more than 10 KV to 20 KV easily generating the atmospheric pressure plasma, the high voltage within the voltage range can cause electric shock, or ignition, smoking or the like due to spark and is extremely dangerous to a human body.
On the other hand, in the safety standard of International Standards IEC60950 (J60950), the input voltage is safe when it is within 60 VDC or its voltage peak value does not exceed 42.4V that is SELV (Safety Extra Low Voltage). Therefore, it is essential to form a configuration that a voltage within SELV is used as the input voltage of the inverter device and a supply power is limited on the input side even if components of the inverter circuit have insulation breakdown for any cause.
Hence, it is required to instantaneously detect the effect of a load in the high voltage inverter device. When overload, insulation deterioration, short-circuit, earth fault, abnormal discharge or the like occurs in a load to which a high voltage is supplied or its power feeding circuit or the like, it is urgent to limit or cut off the power on the input side. If such a state is left as it is, the transformer experiences dielectric breakdown, thereby possibly delivering an electric shock or posing a risk more than that to a human body.
A back electromotive force due to inductance of the transformer increases according to current. Therefore, when the output of the inverter device suddenly becomes no load, a no-load voltage is abnormally generated at power-on of the inverter device, and the abnormal voltage causes breakage of insulation of an insulator or leakage, thereby breaking a component such as a transformer.
Conventionally, in the inverter device with an output power of several mW to several W, the dielectric breakdown and so on of the transformer can be prevented only by inserting a fuse into its input power supply circuit and limiting power.
Further, there is a technique as described, for example, in JP H5-199650A as a conventional abnormality detection device of a high voltage power supply unit. This technique detects a secondary output voltage of a high voltage transformer directly or after dividing the voltage, compares its detection value to a predetermined abnormality determination reference value, judges that an abnormality occurs when the detection value continuously becomes the reference value or lower for a predetermined period or longer, and cuts off the input into the high voltage transformer.
However, in the case that the high voltage power supply unit to be protected is a high voltage inverter device whose output voltage is an alternating-current high voltage exceeding 10 KV, even if it is tried to detect the secondary output voltage directly or after dividing the voltage, there is no resistor standing the high voltage between the output terminals. Even if a plurality of resistors are connected in series for use, the number of the resistors required is several tens or more.
Further, the voltage easily leaks to cause the circuit to malfunction or breakdown. Therefore, the secondary output voltage cannot be easily detected, and it is not easy to provide a protection circuit depending on its detection value.